Hg[CS(NH ₂ ) ₂ ] ₄ (SiF ₆ ): a fluorosilicate crystal with large birefringence achieved <i>via</i> multi-functional group modification

Hexafluorosilicates are known for their short ultraviolet cutoff edges and wide band gaps. However, the low anisotropy of the SiF₆ groups has limited their application in optical functional materials. To address this, a method is proposed herein to enhance the birefringence of hexafluorosilicate through the incorporation of multiple functional groups. Specifically, thiourea CS(NH₂)₂ ligands and highly polarizable Hg²⁺ ions were introduced into the hexafluorosilicate system, leading to the successful synthesis of the first Hg-based hexafluorosilicate birefringent crystal, Hg[CS(NH₂)₂]₄(SiF₆). The structure comprises SiF₆ octahedra interconnected with zero-dimensional Hg[CS(NH₂)₂]₄ clusters <i>via</i> hydrogen bonding. The calculated birefringence at 546 nm is 0.137, which is consistent with the experimental value of 0.145. This performance is comparable to that of the commercial birefringent material α-BaB₂O₄ (0.122@546 nm) and significantly surpasses most previously reported inorganic hexafluorosilicates. This work provides a viable method for improving the birefringence of hexafluorosilicate compounds.